The invention relates to a sensor unit for a water heating system or a water boiler.
Water heating systems or water boilers conventionally are equipped with a manometer, a thermometer, a heating sensor like a NTC-sensor and a mechanical pressure sensor for monitoring and controlling the function. These are separate components mounted at different locations and need relatively high technical effort and a complicated cabling in order to reliably utilise the different information.
A combined pressure and temperature sensor unit as known from EP 0 893 676 A has a variable capacitor within a housing chamber. The capacitor consists of a disk-shaped ceramic substrate portion and a flexible diaphragm portion attached to the substrate portion in axially spaced apart and sealed relation with the diaphragm portion exposed to the fluid pressure and temperature in a sealed pressure chamber. Between the substrate portion and the diaphragm portion a glass sealing is provided. First and second apertures and formed through the diaphragm portion adjacent to the outer periphery. On the diaphragm portion a temperature responsive resistive element like a thick film layer of platinum is disposed in electric connection with probes situated within the first and second apertures in order to sense the temperature. A signal conditioning electronic circuit is situated in a separate housing chamber located above the disk-shaped substrate portion.
A sensor unit as known from EP 0 350 612 A for measuring the temperature and the pressure of a gaseous medium in a suction passage of an internal combustion engine contains in its housing a disk-shaped component having an integrated central membrane carrying a pressure sensor and a temperature sensor. Both sensors are electrically connected to a signal conditioning electronics provided inside the housing. The pressure sensor can be constituted by strain gauge strips or other known measuring elements detecting bending motions of the integrated membrane. The gaseous medium being measured is contacting the disk-shaped component exclusively at the restricted area of the membrane sealed by a central sealing ring. A further sealing ring is placed on top of the upper surface of the disk-shaped element.
Further prior art is contained in U.S. Pat. No. 5,432,372A and U.S. Pat. No. 4,299,117A.
It is an object of the invention to create a sensor unit which can be manufactured with fair costs, has multiple functions in a water heating system or a water boiler, has a reduced mounting requirement and is able to generate precise information reliably and rapidly.
A sensor unit allows simultaneous detection of the water pressure and temperature variations or temperature values. The ceramic support element is in direct contact with the water and comprises an active functional element responding by deformation to water pressure variations. The electronic circuit provides information on the water pressure and the water temperature rapidly and precisely, thanks to the excellent response behaviour of the ceramic support element to temperature and its sensitivity to pressure variations. At least for detecting the water pressure an Application Specific integrated Circuit (ASIC) is used instead of traditional electronics. This equipment assures that the sensor compensates for dimensions in terms of changing with the temperature and linearity for maintaining the output chacteristic within predetermined limits. Gauging is made by writing parameters into an integrated memory. The sensor unit can be mounted in a simple fashion, e.g. on a water boiler. The sensor unit is characterised by a small hysteresis and for that reason is an ideal component for heating applications, particularly boilers. The sensor unit replaces four components as needed in the past, namely the manometer, the thermometer, the heating sensor and the mechanical pressure sensor. This leads to a significant cost reduction.
At least one NTC-sensor (temperature sensitive element operating with a negative temperature coefficient) and/or SMD-chip-thermistor (a temperature sensitive, electronic member operating with positive or negative temperature coefficient and being mounted according to the SMD-technique, i.e. at the surface of ceramic support element) are capable of generating clear signal variations between e.g. 0 and 4 volts in a relatively short response times of less than or equal to 3 ms within a temperature range of e.g. xe2x88x92400 to 140xc2x0 C. in a reliable fashion. Such components, furthermore, are capable of maintaining a drift-temperature-zone point of xc2x10.05% FS/xc2x0 C. at a drift temperature sensitivity of xc2x10.01% FS/xc2x0 C. in connection with a relatively high preciseness (linearity, hysteresis, repeatability) of xc2x11% FS and a zero point tolerance of only xc2x11.5% FS. The electronic circuit zone employed to detect the pressure of the water by means of the active functional element of the ceramic support element should at least be equipped with a resistor, at least one capacitor and at least one microprocessor in order to generate precise signals within a pressure range of e.g. 0 to about 6 bar, while the sensor unit can stand an upper pressure limit of about 12 bar. The ceramic support element offers the advantage of allowing relatively temperature independent pressure measurements.
Expediently the active working element of the ceramic support element may be an integrated membrane which is deformed by the water pressure and which is scanned by electronic members, e.g. by strain gauge elements connected to a Wheatstone bridge. The electronic members expediently are provided at the dry side of the support element. The electric circuit compensates for variations of the Wheatstone bridge caused by temperature variations. Hysteresis cycling and linearity  less than 0.8 F.S can be achieved with an output at zero bar of about 200 mV xc2x11.5 F.S.
It is of advantage when the ceramic support element has the shape of a circular disk containing an integrated and circular membrane in its central region. The material of the ceramic support element may be aluminum oxide, e.g. with a purity of about 96%. The outer diameter of the support element can be about 26 mm and its thickness about 6 mm.
The ceramic support element is secured in its edge region and also is sealed in this region such that no water can reach the electronic circuitry. The inner portion of the support element remains moveable in relation to the fixation of the support element, particularly the limited inner portion defining the membrane and which is actuated by the water pressure. The ceramic support element serves as the carrier for the electronic circuit zones. Expediently a temperature sensitive electronic member is provided outside the limited inner portion at or within the ceramic support element which member is used to detect the temperature of the water. In this case, the ceramic material of the support element serves to transfer the temperature towards the member.
In view to manufacturing it is of advantage to integrate the ceramic support element already equipped with the electronic circuit as a prefabricated chip body during assembly or when injection moulding the housing of the sensor unit.
The sealing can be at least one sealing frame or O-ring made of EPDM plastic material because the material maintains its excellent sealing capabilities within a relatively wide temperature and pressure ranges.
The housing of the sensor unit expediently is, e.g., a unitary or manifold plastic material injection form part consisting of an upper part and a lower part.
For specific conditions the housing can be made at least in part of brass.
The ceramic support element expediently is fixed within a cavity of the housing. The inner channel leads to said cavity in order to actuate the support element with the pressure and the temperature of the water. The securing ring or support element retainer of the housing holds the support element against movement out of the cavity.